Method of anodizing beryllium and product thereof



March 26, 1968 R. D. PITTMAN METHOD OF ANODIZING BERYLLIUM AND PRODUCTTHEREOF Filed oci. 29, 1964 United States Patent Oliice Patented Mar.26, 1968 3,375,179 METHOD OF ANODIZING BERYLLIUM AND PRODUCT THEREOFRichard D. Pittman, Burbank, Calif., assigner to Litton Systems, Inc.,Beverly Hills, Calif.

Filed Oct. 29, 1964, Ser. No. 407,454 4 Claims. (Cl. 204-32) Thisinvention relates to the treatment of metals and, more particularly, tomethods, products, and processes for producing anodic films on berylliummetal.

It has long been known that lberyllium would be a very valuable metal ifit could be properly worked and handled in various manufacturingprocesses. The metal is especially light and strong; it has the mostfavorable strengthto-weight ratio of all light metals. Its conductivityis approximately forty percent that of copper. Beryllium melts -at 1285C., a temperature higher than the melting point of copper. It has anunusually high modulus of elasticity. It has substantially no tendencyto warp, exhibits very little change in dimensions with temperature, isan especially good thermal conductor, and has various other attributeswhich are desirable. Such characteristics make beryllium especiallyuseful in situations where weight, strength, and resistance todimensional change are important, e.g., in precision instruments.

Until recently, however, the manufacturing technology rel-ating toberyllium has been substantially limited. The vlimited technology hasconfined the use of the metal to particular situations. For example,beryllium has a very low atomic number and a low absorptioncharacteristic which make it penetrable by X-rays so the pure metal hasbeen used as windows in X-ray tubes for selective ltration of sof vX-rays, It has also been used in atomic reactors. A number of usefulalloys have been formed with copper, aluminum, nickel, gold, and iron.In general, these alloys are quite hard; for example, beryllium-copperis the hardest copper-'based alloy known.

These few areas clearly do not make full use of berylliums valuableproperties. The main problem in lfurther utilizing beryllium hasbeen-that its crystalline structure is such 4that it, essentially,cannot be worked by conventional methods. For example, the crystalsnormally formed on solidifying from the cold state are so large thatmachining, cold rolling, and other processes were impractical. However,the advent of powder metallurgy (see, The Metal Be'ryllium, White &Burke, published 1955, by the American Society for Metals) allowed therevision of the crystalline structure of beryllium in a manner such thatit may now be practically machined and worked. As a matter of fact, oneof berylliums attractive features is the ease with which it may now bemachined.

A second problem has been that 'beryllium oxide, a normal source andby-product of beryllium, has been thought to be a health hazard like theoxide of lead. However, by practicing well understood precautions, thehazard to health has been substantially eliminated.

Having thus solved the problem of machining beryllium and havingdetermined that the h-azard offered to health might be circumvented, itwould seem that the Way stood open to exploit the metal to its fullest.However, there the technology stopped. Whereas other materials such assteel, aluminum, and the like may be coated and otherwise 'treated inmanners such as to render them corrosion resistant, attractive, heatemissive, and bondable, the methods for so treating beryllium havingbeen severely limited. To some extent this is due to its inherentcorrosion resistance; that is, since the metal displayed thecharacteristic, little study has been directed toward its improvement,This property though useful, is not sufficient for many situations,however. Electroplating processes .have been used for coating berylliumwith a metal. However, these coatings lare conductive and only ascorrosion resistant as the particular metal. At elevated temperatures,no good non-metallic protective coating exists. The problem iscomplicated by the tendency of beryllium to form hard-brittle diffusionphases with the coatings. Thus, though aluminum may be anodized to forma protective coating of aluminum oxide, it has heretofore been possibleto produce only an uneven black coating on beryllium. The coatingproduced is generally so slick that bonding with commercial epoxyadhesives is practically impossible. It is not sufficiently dielectricunless made so thick as to radically affect dimensions. The only reallyusable property of the coating is its `black color which enables it toemit a substantial amount of heat when used in heat-sensitive,temperature-controlled situations. A number of other coatings have beensuggested; but, in general, these coatings attack the beryllium metal,changing the dimensions and weakening the structure so thatv their useis drastically limited.

Thus, the prior art has been unable to provide for beryllium a heatemissive, dielectric, integral, bondable coating `the dimensions ofwhich may be exactly controlled. The use of beryllium has suffereddrastically from this lack. Some researchers have alloyed beryllium withother metals in order to obtain such a coating thereby diluting theeffects of berylliums desirable properties. Others have used mixtures ofother materials with beryllium to the same effect, Therefore, attemptsto anodize have continued.

In attempting to provide a black, dielectric protective coating forberyllium metal, many vknown commercial anodizing processes forberyllium and other metals were evaluated. In general, the commercialcoatings, like aluminum anodizing coatings, build up to cover the basemetal increasing its dimensions. Many of the processes were found toproduce a conductive coating when the coatings were applied to areasonable thickness, one which did not distort dimensions.Non-conductivity, it should be noted, affords protection againstgalvanic corrosion; it also extends the use of beryllium in electricalinstruments. Most of the commercial coatings had to be made so thick toobtain the desired dielectric properties that the dimensions of theparts were appreciably changed. Thus, design allowances for dimensionalchanges were required.

Many of the commercial processes produced coatings with otherundesirable features. For example, none of the `coatings had surfaceswhich facilitated epoxy lbonding to other materials. Many affordedlittle protection from corrosion and abrasion. All in all, nosatisfactory coating was found in the prior art.

To this end, extensive experiments were conducted to devise a newtechnique for coating beryllium metal in accordance with theabove-stated criteria.

I have now discovered a unique method for providing beryllium with asurface coating having characteristics which are almost selectable atwill.` A preferred form of the coating is hard, extremely heatconductive, of selectable color, and exhibits excellent dielectricproperties. .My `method is especially useful because the coating may beformed evenly and consistently to provide the desired 'dimensionsl overalmost any texture of surface of the beryllium metal. Thus, arough-textured beryllium surface, a smooth shiny beryllium surface, astriated beryllium surface, or substantially any other beryllium surfacemay be prepared. Further, the thickness may be limited so that it doesnot appreciably alfect dimensions of the coated part yet provides thedesired characteristics. Bonding to the coating by commercial methodshas .proven quite effective. In addition, the method is veryinexpensive, requires a short time, is substantially.insensitive to thelooseness of controls practiced, and has many other desirable features.

It is therefore a primary object of my invention to cover beryllium witha hard integral layer of material having a selectable dielectricconstant, a selectable coloration, and a consistent selectablethickness.

Yet another object of my invention, in a primary sense, is to anodizeberyllium metal.

Another object of my invention is to cover beryllium metal evenlywithout changing its dimensions materially.

An additional object of my invention is to provide coatings forberyllium metal by easily controlled processes.

It is another object of my invention to anodize beryllium to form ahard, heat-conductive, dielectric coating of even thickness thereon.

An additional object of my invention is to anodize beryllium with acoating of material having a thickness which may be accuratelycontrolled to minimum dimensions.

Yet another object of my invention is to anodize beryllium with coatingsto which other materials may be easily bonded by commercial methods.

In realizing the above objects, I have devised processes which areespecially inexpensive and require a minimum of control. For example, ina preferred process the material may be cleaned by substantially anymethod desired. Thus, the surface of the metal may be prepared in amanner such that it is dull, bright, or has almost any other desiredcharacteristic. The coating which forms may be made so thin that the nalsurface is substantially identical to the original and exhibits itscharacteristics. After surface preparation, the part to be coated isconnected to a positive source of DC potential; the negative terminal ofthe DC source is connected to an inert cathode material, eg., thecontainer holding the coating bath. The part is immersed in the coatingsolution; and the circuit is energized, either before or after theimmersion. Depending on the time, temperature, and material of which thebath is composed, a coating is formed on the beryllium having aparticular color, a particular thickness, and others of theaforementioned characteristics, as desired.

The process, of course, depends upon the bath. I have determined thatthe most useful bath :from the standpoint of characteristics includessodium borate and sodium hydroxide proportioned to provide an excess ofhydroxyl ions. When energized at a low initial voltage for periodsgreater than fifteen minutes, the desired black, bondable, dielectriccoating is produced.

It should be especially noted, before proceeding to a more specificdescription of my processes, that the baths which I use for the anodiccoatings for beryllium metal are varied yet fall within substantially aclass of solutions. Thus, once the key to selecting the bath and theprocess for the coating is explained, it will be obvious to othersskilled in the art just how to prepare additional baths for coatingberyllium metal falling Within the scope of my teaching.

The objects, features, characteristics, and advantages of the processesof my invention will be better appreciated from the following specificdescription in which the single figure is a pictorial representation ofthe physical components and arrangement necessary for accomplishing thecoating process of my invention.

As pointed out above, various coating processes are available forberyllium metal. For example, a number of anodizing baths are capable ofturning beryllium black. However, the baths do not produce coatings towhich commercial high strength adhesives such as epoxy bond well. Theconventional bath Ifor blackening the exterior of beryllium metal is achromic-acid/nitric-'acid solution which may contain as much as 701 Baumnitric acid (concentrated) and various other additives. The coatingproduced is black, but slick and uneven. The epoxy adhesives used incommercial bonding processes do not adhere well to the coat due to itsslickness. The coating does adhere well to the beryllium metal which itprotects.

However, in thin layers it is not sufficiently dielectric.` Further, inthick layers the coating is quite uneven. In order to provideappropriate overall protection of the beryllium nretal, the coating mustbe made so thick that general -dimensions of the material are changed.This is due to the fact that these coatings build up to cover thevsurface of the metal.

In view of the inadequency of the available anodizing baths forberyllium, experiments were conducted in the hope of discovering a bathwhich would provide the desired characteristics. It should be especiallynoted that a cornmercial sodium hydroxide bath was tried but provedincapable of producing the desired characteristics. A boric acid bathwas also tried but appeared unsuccessful though indications were thatthe coating produced was slightly resistive; an ammeter used in circuitwith a source of DC power showed a definite current drop as the coatingformed. However, the coating produced Was white and powder-like.

In view of the fact that the boric acid bath showed some .possibility ofproducing at least one of the desired characteristics, an attempt wasmade to get more boric acid into the bath solution. To this end, sodiumhydroxide (which had previously proved unsuccessful) was added. When thesolution was at a point in which four gram*- moles of boric acid weremixed with three gram-moles of sodium hydroxide in a liter of water, aheavy blue-whitegray coating was produced. rPhe coating was quitedielectric and would not pass current detectable to my instruments. Thiswas the first experimental success.

However, the coating abraded quite easily and had the wrong color forthe planned use. Postulating that beryllium oxide formed by theanodization lent the gray to the coating, the sodium hydroxideconcentration was increased and the boric acid concentration wasdecreased. This lessened the borate ion in solution and increased theberyllium oxide in the coating. When the concentration of the bathreached four gram-moles of sodium hydroxide and three gram-moles ofboric acid, the coating turned black. This was the second success in theexperiments. Thereafter, I found that by lowering the boric acid contentof the solution a thinner film was possible. After considering the bathVcontent, I decided to use sodium borate instead of boric yacid and toadjust the solution to fgive different pH concentrations. Thisarrangement proved the key to success in anodizing beryllium metal toobtain the desired properties.

In each case, it was determined that a coating formed on the berylliummetal. However, the properties of the coating varied depending on the pHand pOH of the solution. For example, the coating which had formed whenan excess of boric acid was included in solution, though dielectric, waswhite, thick, and non-adherent whereas the coating which formed when anexcess of sodium hydroxide was included in the solution was dielectric,black, thin, and adherent. Thus, the characteristics of the coatingclearly depended on the particular percentages of the acid and base usedin the solution. After some experimenting, it was determined that thesolution best adapted to produce the characteristics desired was one inwhich all acid was neutralized and excess hydroxyl ions were present.For example, a neutral solution of sodium borate to which was addedsodium hydroxide, from a relatively minor amount to a saturatedsolution, would prove most useful. The various experiments demonstrated,however, that with different concentrations of sodium hydroxide or boricacid, with different initial voltages, with different temperaturecontrols, and soon, the bondability of the coating could be varied, itscolor and hardness could be changed, and its other properties could beoperated upon almost as desired. f

In brief, the process of this invention amounts to an electrochemicalformation of an oxide film of beryllium through the combination of ionsfrom the bath solution with the beryllium metal anode. In the figure, adirect 5 current source is provided for biasing l'he beryllium metalanode to a positive potential. The other terminal of the biasing sourceis connected to another electrode to define a cathode. In a particularembodiment, the cathode is the stainless steel tank used to hold thebath solution. When current is passed through the bath solution shown inthe tank of the figure, the coating immediately begins to form, Aninitial voltage as low as two volts may be used; howexample, berylliumoxide is normally considered to be a White material. However, I havebeen able to form a blue coating, a dark grey or black coating, a tancoating, and a coating of other colors. I feel that the amounts of boronin the coating and other compounds of both beryllium and boron arepresent and affect the characteristics. The following tables listvarious properties of the coatings formed for particular solutions used:

TABLE I TEMPERATURE ADJUSTMENTS OF BATH SOLUTION [Ratio: Boric Acid3/Sodium Hydroxide 3.5]

V; A; A2 T1 Tg T1 A ASF V.B. Color 14.5 2. 6 0. 12 2506 2506 .1 T3 3. 974. 32 O Light Gray. 14. 2. 8 0. 13 2500 2501 1 4. 23 4. 46 0 Do. 13. 82. 9 0. 195 2502 2503 .1 3. 75 7. 49 0 Do. 14. 2 3.0 0. 137 2502 2502 .l'la 4. 48 4. 32 0 Do. 14. 4. 0 0.250 2500 2500 .1 T3 4. 72 7. 63 0 LightBrown. 14. 5 4. 0 0. 200 2502 2503 .1 3. 99 7. 2 0 Dark Brown. 16. 0 5.8 0.260 2495 .24955 .1 Ta 3. 7S 8. 78 0 D0. 15. 0 5. 0 0.310 2502 2503.1 4. 15 10. 66 0 D0.

Vr=Initial voltage. Tg=Thickness of coated sample. V2=Final voltage.T3=Thickness of coating in mils. A1=Initial amperage. A=Area in squareinches. A2=Final amperage. V.B.=Voltage breakdown. T1=Thickness of baresample. ASF=Amperes per square foot.

TABLE II.-TEMPE RATURE ADJUSTMENTS OF BATH SOLUTION [Ratio: Boric AcidS/Sodiurn Hydroxide 3.75]

Tgnlip., V1 V1 A1 A.; T1 T2 T3 A ASF V.B. Color 7. 0 13. 7 3. 2 245 25022503 .1 3. 87 9. 07 0 Tan. 7. 0 14.5 4. 8 455 2496 2497 .1 4. 51 15. 6 0Dark Tan. 7. 0 14. 0 4. 0 305 2488 2438 .1 Ta 3. 2 13. 7 0 Golden Brown.7. 0 13. 5 4. 5 620 2493 2493 .1 Ta 4. 03 22. 2 0 Black Violet. 7. 014.5 5. 6 720 2429 2429 .1 T3 4.03 25. 6 0 Metallic Blue. 7. 0 13. 7 6.0 l. 120 2459 2459 .1 Ta 4. 28 37. 6 0 Olive Drab. 7. 0 13.8 6. 0 1.12464 2464 .1 T3 3. 56 44. 5 0 Iridescent Brown. 7.0 13. 0 6.9 2. 2 24912492 .l 4. 21 75. 3 0 Bluish Black.

V1=Initial voltage. T2=Thickness of coated sample. V2=Final voltage.Ts=Thickness of coating in mils. Ai=1nitial amperage. A=Area in squareinches. A2=Final amperage. V.B.=Voltage breakdown. Tr=Thickness of baresample. ASF=AInperes per square foot.

TABLE IIL-INITIAL VOLTAGE ADJUSTMENTS OF BATH SOLUTION [Ratio: BoricAcid 3/Sodium Hydroxide 3.75]

V1 Vr A1 A; T, Tr T3 A ASF V.B. Color 2. 0 4. 2 290 080 2501 2501 3. 9710. 5 0 Light Gray.v 4. 0 5.0 1.0 750 2496 2496 4. 23 34.0 0 Gray. 6. 08.5 3. 2 l. 8 2500 2500 3. 75 29. 9 0 Black (periphery). 8.0 16.0 6. 6 82500 2500 4. 48 62.0 0 Dark Brown.

Columns have the same meanings as in the above tables.

ever, better properties will be obtained where the initial voltage ismaintained at five or six volts. However, higher voltages may be used.In a particular embodiment, the temperature of the bath need only bemaintained between 70 and 180 Fahrenheit. Current is applied until therequired thickness of coating is formed (e.g., from fifteen minutes-onehour). In one exemplary bath, a neutral solution of sodium borate ismixed with a small amount of sodium hydroxide to provide an excess ofhydroxyl ions. An especially useful solution contains sodium borate andsodium hydroxide in a ratio of 2.69 gram-moles to 0.77 gram-moles. As apotential is applied, oxygen is produced at the anodes, forming an oxidecoating on the beryllium metal. By controlling the temperature of thebath (through the temperature control shown on the figure), the durationof the process, the initial voltage applied to the voltage control, thepH of the solution, and by selecting an appropriate surface treatmentfor the beryllium metal, an infinite range of colors, thicknesses,surfaces, and conductivity properties may be obtained.

I believe the coating actually comprises beryllium oxide held in acovalent bond with boron. The structure of the beryllium is such that itmay form a n-umber of diierent covalent bonds when different electronsare shared with oxygen thereby providing the different characteristics.For

In a preferred process in yaccordance with the invention, the bathcomprises sodium b'ora'te having a consistency of 177 grams per literand sodium hydroxide having a consistency of 30 grams per liter. Thebath is used at an operational tempera-ture of .to 120 Fahrenheit. Aninitial voltage of six volts is applied. The process continues for aperiod of yone hour. No particular preparation is necessary for thispreferred bath though normally one of the chemicals is dissolved insolution and the bath brought to tempera-ture before the other is added.

The surface of the material is cleaned in a manner =to provide thedesired nal nish. In this regard it should be noted that no specialcleaning techniques are required for this bath. This is in contrast `tothe cleaning techniques necessary for anodizing aluminum in the standardchromic acid bath.

The material to be anodized is placed in the lbath solution as t-heanode. The electrical connection to this material is normally insulatedfrom the -bath solution to prevent an inappropriate anodizing fromoccurring at the connection itself. The 4conductor 4to t-he `anode maybe coated with a plastic material which is unaffected -by the anodzingbath; or an electrical connection of the same material, beryllium, maybe used. The particular operation is substantially insensitive to minorvariations in parameters and produces a coating which is black,dielectric (measures 30G-500K ohms resistance), thin, bondable, and hasa selectable surface and dimensions.

Certain of these characteristics are of especial importance. Forexample, `the lack of special cleaning techniques eliminates one of thevery expensive steps of most anodizing processes. It also allows thesurface to be prepared as one desires for the final coating. Thisresults because the coating, though dielectric, may be made so thin thatthe dimensions remain substantially the same. A typical dimensionalchange is 0.0002 inch. Thus, if a very smooth mirror-like surface isprepared on the beryllium metal, the same surface will `appear on thefinal coating. On the other hand, if a stippled surface is prepared, astippled surface will result for the coating. Also of especialimportance with regard to the thickness and nish of the coating is thefact that dimensions need not be programmed for the coating to allowcorrections for add on during the process. The coating is actuallyformed with the surface beryllium and is so thin thatV essentially thesame dimensions and surface remain after the coating as those of theoriginal material. The same surface remains though its constituency haschanged. This is especially important in manufacturing preciseinstruments.

The multi-colored aspect of the coating allows a selection .fordecorative, industrial, or other purposes. For example, the originalblack coating was chosen because a heat emissive exterior was desiredfor the particular instrument. On the other hand, it might be desirableto identify certain port-ions of a complex process by the color of thematerials used. With this bath, identifications could be accomplishedlby color. It should be particularly noted that the coloring isaccomplished in the bath without the necessity of a dyeing process, asis usual 'm aluminum anodizing. Thus, another step is eliminated from`conventional processes. Further, the entire coating is of the selectedcolor rather than just the surface portion so that the abrasion of thematerial is not so likely to cause flaws in the color as with aluminumanodized material.

The wide temperature range by which it is possible to reachsubstantially the same results is especially valuable in providing aprocess which may be run by industrial workers. This advantage`obviously adds to the economies of the proposed process. On the otherhand, the low power and the short time necessary for the anodizingmaterially reduce processing costs.

As pointed out before, the conductivity of the coating may be varied asdesired. In the particular instrument for which the coating wasoriginally prepared a dielectric coating was desired. Such a dielectriccoating is especially useful in preventing electrical shorts and leakagein precision instruments and substantially eliminates galvanic corrosionwhich may tend to effect rapid deterioration of precision instrumentsusing some prior art coatings.

The bondibility of the surface produced by the preferred process is alsoespecially desirable. The prior art techniques for producing a blackcoating on beryllium evidently were not gauged with modern productionprocesses in mind, and the slick coating formed by some of the prior arttechniques limited the techniques by which beryllium parts could be madeto adhere to other elements. With commercial epoxy adhesives designatedFAS and LCA 4/ 9, a minimum block shear strength of 6100 p.s.i. wasobtained.

The coating may be made both abrasion and corrosion resistant. Forexample, when subjected to a 5% salt water solution and to like vapors,the coating showed no effect after 120 hours though beryllium metal wasseverely etched after the same period.

Having shown the techniques for selecting the various combinations usedin the bath and the dilerent controlling elements in the process, thoseskilled in the art will be able to devise numerous other baths,processes, and coatings. For these reasons, the invention is to beconsidered as restricted only within the scope of the appended claims.

What is claimed is:

l. A method of coating beryllium comprising the steps of immersing apiece of beryllium in an aqueous alkaline bath having a free hydroxylions and borate ions, placing a potential between said piece and asecond terminal sufficient to generate a current in saidV bath whereinsaid piece is anodic, and anodizing said piece to produce a coatingthereon.

2. The product produced by the process of claim 1.

3. A process of anodizing beryllium comprising the steps of preparing adesired surface on the beryllium metal by a selected cleaning technique,immersing the beryllium metal in an aqueous alkaline bath of an alkalimetal borate solution containing free hydroxyl ions, connecting a sourceof potential between the beryllium metal as anode and a cathode, andpassing current through the electrolyte bath by means of the source fora selected time.

4. A method for selectively coloring beryllium comprising the steps ofpreparing an aqueous alkaline bath comprising sodium hydroxide and boricacid having free hydroxyl ions, immersing the beryllium in the bath,connecting the beryllium to a positive terminal of an electrical source,applying a selected voltage between the beryllium and another point inthe bath sufficient to pass current through the bath, and maintainingthe current for a selected period.

References Cited UNITED STATES PATENTS 2,373,273 4/1945 SZiklai.2,871,425 l/l959 Burnham 204-56 XR 3,180,807 4/1965 Quinn 204-56 XR3,276,974 10/1966 Tyson 204--56 XR FOREIGN PATENTS 813,994 5/1959 GreatBritain.

HOWARD s. wiLLrAMs, Primary Examiner.

ROBERT K. MIHALEK, Examiner.

G. KAPLAN, Assistant Examiner.

1. A METHOD OF COATING BERYLLIUM COMPRISING THE STEPS OF IMMERSING APIECE OF BERYLLIUM IN AN AQUEOUS ALKALINE BATH HAVING A FREE HYDROXYLIONS AND BORATE IONS, PLACING A POTENTIAL BETWEEN SAID PIECE AND ASECOND TERMINAL SUFFICIENT TO GENERATE A CURRENT IN SAID BATH WHEREINSAID PIECE IS ANODIC, AND ANODIZING SAID PIECE TO PRODUCE A COATINGTHEREON.
 2. THE PRODUCT PRODUCED BY THE PROCESS OF CLAIM
 1. 3. A PROCESSOF ANODIZING BERYLLIUM COMPRISING THE STEPS OF PREPARING A DESIREDSURFACE ON THE BERYLLIUM METAL BY A SELECTED CLEANING TECHNIQUE,IMMERSING THE BERYLLIUM METAL IN AN AQUEOUS ALKALINE BATH OF AN ALKALIMETAL BORATE SOLUTION CONTAINING FREE HYDROXYL IONS, CONNECTING A SOURCEOF POTENTIAL BETWEEN THE BERYLLIUM METAL AS ANODE AND A CATHODE, ANDPASSING CURRENT THROUGH THE ELECTROLYTE BATH BY MEANS OF THE SOURCE FORA SELECTED TIME.